The present invention relates to a multicolor image forming method of the kind sequentially developing a plurality of latent images electrostatically formed on an image carrier by toners of respective colors and transferring the resulting toner images to a recording medium at the same time. More particularly, the present invention is concerned with a bicolor image forming method which transfers toner images formed on the image carrier in two colors to a recording medium at the same time.
A bicolor image forming method is extensively practiced with a copier, facsimile transceiver, printer or similar apparatus. The method consists in forming a toner image of first color on an image carrier by first charging, exposure and development, forming a second toner image on the image carrier by second charging, exposure and development while holding the toner image of first color, and transferring the toner images of first and second colors to a recording medium at the same time. Usually, the toner image of second color is formed by a noncontact type developing unit, i.e., a developing unit having a developing roller spaced apart from the image carrier and using a nonmagnetic toner, so that it may not disturb the toner image of first color existing on the image carrier, as disclosed in, for example, Japanese Patent Laid-Open Publication Nos. 60471/1988, 63061/1988, and 85578/1988. The problem with the conventional method is that when a bicolor image forming procedure is repeated over a long period of time, the toner image of second color becomes impure and thereby degrades image quality. This stems from the fact that the toner forming the toner image of first color on the image carrier unexpectedly flies into the developing unit storing the toner image of second color when of forming the toner image of second color.
Bicolor image forming methods elaborated to eliminate the above problem are disclosed in Japanese Patent Laid-Open Publication Nos. 7252/1986, 294579/1988, 294580/1988 and 123069/1988 as well as in Japanese Patent Publication No. 45916/1989. None of them, however, can ensure stable bicolor toner images over a long period of time.
On the other hand, Japanese Patent Laid-Open Publication No. 48683/1990 proposes a bicolor image forming method of the type forming toner images of first and second colors on an image carrier by toners of first and second colors which are opposite in polarity to each other, uniformizing the polarity by a pretransfer charger, transferring the toner images at the same time, and removing the toners remaining on the image carrier after the image transfer. Although this type of method enhances the cleaning efficiency, it has various problems left unsolved, as follows. The charging effected by the precharger increases the charge potential of the toner of the same polarity as the charging, i.e., lowers the charge potential of the other toner of the opposite polarity relative to the former. As a result, the two toner images are charged in different amounts at the time of simultaneous transfer. It follows that although one of the two toners may be efficiently transferred to a recording medium, the other toner is transferred only defectively or with an undesirably enhanced edge effect. In this situation, it is difficult to set up an adequate potential for transferring the two toner images at the same time and, therefore, to provide images with constant density. Regarding the cleaning step, despite that the pretransfer charger lowers the potential, the toner of the same polarity as the pretransfer charge carries a great amount of charge and, therefore, constitutes a heavy load. Such a load is apt to effect the removal of the remaining toner to be performed by, for example, a blade or a brush. Moreover, this kind of image forming procedure is not practicable without resorting to a bulky, complicated and, therefore, expensive apparatus.